The present invention relates to a record stylus for capacitance disks in which signals are recorded in the form of geometric variations along each of spiral or concentric tracks and detected by the stylus in the form of capacitance variations.
A record stylus of the capacitance detection type comprises an insulative body such as diamond or sapphire and an electrode deposited on a rear surface of the insulative body. The electrode, the width of which is substantially equal to the width of the track, extends from the bottom of the insulative body toward its upper end for connection to an external processing circuit. The thickness of the electrode is typically of the order of 2000 to 3000 Angstrom to pick up signals which are recorded in the form of minute pits having a minimum length of 3000 Angstrom in the direction of travel of the disk and a depth of 3000 Angstrom. To detect capacitance variations a high-frequency bias, typically at 1 Gigaherz, is applied to the stylus electrode with respect to a conductive film embedded in the disk record. A shortcoming inherent in the conventional capacitance stylus is a problem of wear that occurs on the bottom edge of the electrode such that it diminishes in length as much as 500 Angstrom with a resultant increase in spacing between the electrode and the record surface. Conventional capacitance detection type record styli thus degrade as a function of time in signal output level, signal-to-noise ratio and in high frequency response, and crosstalk eventually occurs between adjacent tracks.
Various attempts have hitherto been made to mitigate this wear problem on the assumption that the wear is caused by the difference in material's hardness and abrasion resistance properties between the insulative support and the electrode. One approach involved sandwiching the electrode between two insulative supports so that the rear face of the electrode is not exposed to air. Another approach involved the provision of an insulative protective layer such as aluminum oxide or silicon dioxide on the electrode. However, satisfactory results have not yet been obtained.